VENICE, Italy (Reuters) - Italian authorities were investigating on Wednesday the cause of the horrific crash of a bus carrying foreign tourists on the outskirts of Venice which killed 21 people including several children and injured 15 others.

The electric bus crashed through the guardrail and off an overpass late on Tuesday in the Mestre district, slamming into the ground more than 10 metres (33 ft) below and catching fire.

Five Ukrainians, a German and the Italian bus driver were among those killed, Venice's prefect Michele Di Bari, the local representative of the interior ministry, said on Wednesday. He did not comment on the nationality of the others killed.

The accident happened shortly before 8:00 p.m. (1800 GMT) on a straight and normally very busy road that connects Mestre to the historic centre of Venice and runs alongside a railway.

"We presume the driver may have fallen ill," Veneto regional president Luca Zaia told Rtl 102.5 radio. Witnesses' accounts and CCTV footage might give additional clues, he said.

Another four Ukrainians, two Spaniards, two Austrians, a German, a Croatian and a French person were among those hurt in the crash, Di Bari said. The bus had been ferrying the tourists back to a campsite in nearby Marghera after a day out in Venice.

Ukrainian President Volodymyr Zelenskiy spoke on Italian television on Wednesday to express his condolences to the families of all the victims.

'GET MY DAUGHTER'

People living nearby said they had heard a loud bang and had rushed to the scene to try and help.

"There was a woman speaking English and crying. While I was pulling her out, she said "get my daughter, get my daughter," Godstime Erheneden, originally from Nigeria, told Corriere della Sera newspaper.

"It was a small child, I think she was two years old. She was unconscious and I think she was dead. I am very upset, she is the same age as my son," he added.

Number plate checks indicated that the bus was an electric vehicle made by a Chinese manufacturer. Its wreckage was still being monitored for fire risks from its batteries, firefighters said on Wednesday.

The head of the company that operated the bus, Massimo Fiorese, told Corriere della Sera he had seen video footage of the moments before the accident.

It showed the single-decker bus slowing down and appearing "almost stationary" when it crashed through the guardrail and fell, he said.

"I think the driver had an illness, because otherwise I can't explain it," Fiorese said, adding that the driver had started his shift less than two hours before the crash.

Footage of the accident site showed that the overpass was equipped with an old, relatively thin metal guardrail, rather than newer, sturdier concrete versions.

Yellow flowers had been tied to the mangled rail on Wednesday and the site sectioned off behind concrete blocks.

(Writing by Alvise Armellini and Keith Weir, Editing by Bernadette Baum, Gavin Jones and Gareth Jones)